Galvanometer with resilient damper



iatented Dec. 27, 1949 I GALVANOMETER WITH RESILIENT DAMPER Glenn L.Dimmick, Haddon Heights, N. 1., asai'gnor to Radio Corporation ofAmerica, a corporation of Delaware Application October 4, 1946, SerialNo. 701,099

This invention relates to sound recording apparatus, and particularly toa modulating device known as a galvanometer, such as is used in therecording of sound on film.

Galvanometers of the general magnetictype of the present invention areknown, reference being made to my prior United .States Patents No.1,936,833 of November 28, 1933 and No. 2,189,- 311 of February 6, 1940.Although galvanometers of the type shown in these patents have been usedin commercial sound recording systems, they have been subject to aslight non-linear relationship between the input voltage of the signaland the vibration amplitude of the modulating element such as themirror. The present galvanometer is of the general overall constructionof my prior galvanometers, but the structure and circuit have beenmodified to obtain an improved overall performance.

Specifically, the new construction of the gaivanometer reduces the oddand even harmonic distortion of the wave shape of the signal, reducesthe detrimental effects of hysteresis, and provides a more faithfulcorrespondence between the motion of the mirror and the input signalvoltage. These results have been accomplished by improving the magneticcircuit or path, by providing' a special form of armature, and byvarying the. type of electrical circuit both within and without thegalvanometer.

The principal object of the invention, therefore, is to facilitate therecording of sound on film.

Another object of the invention is to provide an improved system formodulating a light beam.

A further object of the invention is to provide an improved magnetic andelectrical light beam modulating galvanometer.

A still further object of the invention is to provide a sound recordinggalvanometer having a substantially uniform motional response throughoutthe major portion of the audio range.

Althou h the novel features which are believed to be a characteristic ofthis invention will be pointed out with particularity in the appendedclaims; the manner of its organization and the mode of its operationwill be better understood by referring to the following description,read in conjunction with the accompanying drawings forming a parthereof, in which:

Fig. 1 is aperspective view showing the principal elements of arecording galvanometer embodying the invention;

Fig. 2 is a schematic circuit diagram of and for the galvanometer;

10 Claims. (01. 171-95.)

Fig. 3 is a detail view of the armature used in the galvanometer;

Fig. 4 is a graph illustrating the adjustments made to obtain thedesired frequency response:'

and

Figs. 5 and 6 are graphs showing the improvement of the galvanometer ofthis invention over my prior galvanometer constructions.

Referring now to the drawings, in which the same numerals identify likeelements, the galvanometer is provided with magnets 5 and U- shaped polepieces 0 and I, an armature 9 being clamped between two non-magneticspacers i0 and Ii and held between the ends l3 and ll of the pole pieces6 and I. As in my prior con-- bifurcated support 24 which is mounted onthe pole pieces 6 and I. This ribbon serves to hold the V-block '20 onthe knife edges at the end of the armature. The ribbon 23 is ofberyllium copper approximately one mil thick and sixteen mils wide. 20,and, due to rigidity in the lengthwise direction, it prevents the block20 from executing sidewise or lateral vibration in response to thelateral vibration of the armature. When the armature vibrates laterally,this motion is converted into torsional vibration of the V-block 20,since the ribbon 23 is relatively flexible in torsion.

In my prior constructions, two coils were wound or positioned around thearmature in the space within the pole pieces 6 and I. In the presentinvention, a single coil 26 is provided at this point around thearmature, the coil being tapped; In one form, the coil may be wound with1350 turns of No. 42 wire, with the tap taken off at 675 turns, although1400 turns may be used with the tap at 700 turns. As shown in Fig. 2,the terminals of the coil are connected to the output of a noisereduction amplifier 3| consisting of a rectifier and direct currentamplifier as described in U. S. Patent No. 2,361,451 of October 31, 1944and U. S. Patent No. 2,419,001 of April 15, 1947, while one terminal ofthe coil and the tap are connected to a recording am- It passes over thetop of the block sheave is the use of a diirerent type of damper. Theprevious dampercomprised a U-shaped element between the ends ot whichand the armature were positioned tungsten loaded rubber elements, asshown in my above mentioned Patent No. 2,189,311. The 'new galvanometeruses, in place of this prior damping element, a lineqasnper as shown at33. which is of the type ?disclosed in copending application of Edwardw. Kellogg. Serial No. 611,890, filed August 21, 1945. This dampercomprises a strip of rubber-like resilient material, preferably tungstenloaded neoprene substantially .70 inch in length and .06 inch square or.016 inch in diameter ii round. One end of the damper is rigidlycemented to a disc 34 which is soldered to a .01 inch diameter rod 3!partially inserted into the damper and mounted at the end of the V-block2,0 and the other end of the damper is free. This line damper is mountedin this manner, since a greater torsional movement is obtained at thispoint than at any other moving point oi the structure. In the past,dampers have been placed around the armature 9 where the vibration is avery small fraction of a mil which made it difllcult to get sufllcientdamping with the small components within the coil. Since the length ofthe free portion of the armature is approximately .240

inch and the distance between the groove and' strip 23 is approximately.020 inch, the vibrating elements act as a step-up transformerconvertvalue of mils to a value of mils.

ing the relatively small torsional vibrations oi the armature about itsclamped end into relatively large torsional vibrations of the mirror 2!and block 20 about the center of strip 23. The whole galvanometer isrelatively small, so the torsional line must, of necessity, be small soit is a definite advantage to couple the small torsional line to thatportion of the vibrating mechanism having the greatest amplitude oftorsional vibration. v

The use of this type of damper 33 has not-only reduced the mechanicalresonant peak in amplitude, but the resulting characteristic is moreuniform over the critical frequency range. That is, as shown inmypatent, this prior type of damper introduced a double humpedcharacteristic, while the present damper produces a smooth continuationof the characteristic. vFurthermore, this damper is relativelyunail'ected by temperature This result is illustrated in Fig. 4, as

changes. I indicated by the upper dotted and solid char acteristiccurves A and B.

. As mentioned above, one of the disadvantages.

of prior galvanometers' was in the' introduc- Y 'tion' of odd harmonicdistortion of the. wave shape.

'Ihishasbeendue chiefly to magnetic. saturation of the armature. ".Toavoid this condition, the

armature has been made wider to increase the flux carrying capacitythereof. Indoing the Stiffness of the armature was increased indirectproportion to the increase in width but this unby a novelmodification oi the magnetic path. This includes increasing the lengthof the air aps in the magnetic path which increases the permissibletolerances in armature placement, spacer thickness and thenon-uniformity of magnetic material. It is well known that the presenceof direct current flux increases the amount of even harmonic distortionin an iron path. This detrimental effect has been greatly reduced byincreasing the air gap length to dilute the effect of the iron. Thisalso reduces the efiect of hysteresis on the vibrating system thuspermitting the mirror to return to its zero or biased position with agreater degree of precision. Thus, in the present construction, the airgap at the clamping end of the armature has been increased from 10 milsto 50 mils by increasing the thickness of the non-magnetic spacers l0and II. The front air gaps at the points I3 and I4 have been alsoincreased from their former The total amplitude of vibration of thearmature 9 is approximately 0.5 mil, this deflection occupyingapproximately only of the air-gap length provided. The'lengthening ofthese two air gaps increased the amount of air reluctance included inthe magnetic circuit by the ratio of 3 to 1. The amount of the reductionof both even and odd harmonic distortions by this new construction isillustrated in Figs. 5 and 6, under both biased and unbiased conditionsas described. In Figs. 5 and 6, the solid lines represent the distortionof the galvanometer of the invention, while the dotted lines representthe distortion of my prior galvanometer structures.

The fact that the air gap reluctance was increased 3 to 1 meant that, inorder to obtain the same amount of alternating current flux in thearmature, the magnetomotive force in the coil had to be increased by 3to 1. If the coil were left the same as before, and the current steppedup 3 to 1, thenecessary flux could be obtained, but the heat developedin the coil would go up nine times, and this would, of course, overheatthe coil. To avoid this difliculty, the amount of copper in themodulating coil was increased, thus making it more eflicient, but alsoincreasing its the ratio of resistance to inductance to the valuerequired for the proper frequency response, it

was necessary to add 500 ohmsoi resistance in seri'es with thegalvanometer, but the resistancev was placed outside the galvanometer sothat most 7 of the heat developed did not heat the galvanometer. Thisresistance is shown at 37 in Fig. 2, I

the condenser 38 being for the purpose of preventing biascurrent fromfeeding into the recordviding pole pieces which provided higher fluxdensities in the air gap. This was accomplished by using two percentvanadium permendur in the form of .011 properly annealed.

, The final shape of the armature is as shown in Fig. 3, wherein theclamped portion is shown at I! and the vibrating section is shown-at[5,- the Even harmonic distortion-has been" caused ing amplifier,itsvalue being large, such as 10 mi., to provide a goodfrequency'response down to cycles. 4

As mentioned above, the prior galvan'ometers had both a modulation coiland a bias coil, while the new galvanorneter has a single coil, whichmust producethree times the imagnetomotive force to make up for thewider air gap. The

singlecoihhowever. can serve for both m'odulation and noise reduction,inasmuch as the noise dotted linesrepresenting'the width of the arma-Itures in myprior galvanometersreduction current is maximum whenthe'signal current is minimum, and vice-vcrs a.- However, in order toproduce a uniform frequency response:

characteristic as shown by curve C in Fig. 4, it

isn'ecessary'that the desired ratio of inductance I V J to resistancevbe maintainedb "I'he ordinatesofcurve D in Fig; 1 represent the currentwhich is obtained through the proper inductance-resist a m-1s ancecombination when constant voltage is applied. The ordinates 'of'curve'Brepresent the amplitude at each'frequency. when a constant current isapplied to the galvanometer. Thus. in order to produce the resultinguniform characteristic shown by curve C, theordinates must be equaltothe products of theordinates of curve B and curve D. The proper ratio ofinductance to resistance was obtained by. using the resistor as shown at31 which, in a .galyanometer having the number of turns referred toabove, had a value of 500 ohms.

To explain further the relationship between curves B, C, and D. thecurve marked shows the relationship between amplitude and frequency whenconstant voltage is applied to the electrical network associated withthe, galvanometer. This is the way the galvanometerwould be used inpractice, and it is, therefore, desirable for the curve to be as nearlysmooth and fiat as possible when operated under normal conditions. CurveD represents the variation of current in the galvanometer with frequencywhen constant voltage of the galvanometer, especially in the mid-rangefrom 2000 to 5000 cycles. The capacitor alone can be made to neutralizethe inductance of the coil at any desired frequency and, therefore,permit the frequency characteristic to be improved in this range. Byemploying a combination of capacitors andseries resistors, it was foundthat still better control of the frequency characteristic in the midfrequency range could be obtained by properly choosing the values ofthese two components.

Therefore, the above described new construction has resulted in a lightbeam modulating galvanometer which, when used in a sound recordingsystem, improves the quality or fidelity of the recorded {sound track.

I claim as my invention:

1. A galvanometer comprising means for forming a magnetic field, avibratory armature mounted in -said field, said armature terminating ina knife edge, a coil mounted around said armature,

is applied to the input of the associated electrical I network. Curve 13represents the variation in the amplitude of galvanometer deflectionwith frequency when constant current is passed through the galvanometer.The-reason that the curve is of this shape is explained in detail in myarticle entitled "Galvanometers for variablearea recording," publishedin the Journal of the Society of Motion Picture Engineers, volume XV,No.4, 1930, pages 428 to 438. Fig. 2 of this article shows the responsecurves for a damped vibrating mechanical system where various degrees ofdamping are employed. The theory upon which the curves are based isgiven in the paper.

For instance, if Y represents the ordinates of curve D, then the curveis represented by the relation where A is the amplitude of vibration ofthe galvanometer and I is the current through the modulation coil. If wemultiply these two equations together, we obtain an equation whichrepresents the curve shown in 0, whose amphtude is In other words, curveC represents the amplitude obtained when constant voltage is applied tothe input of the network associated with the galvanometer.

Across the winding 26 a shunt circuit is shown, comprising a resistor 40and a condenser ll having a value of 1200 ohms and .005 mf.,respectively, the purpose thereof being to improve the frequencyresponse characteristic by partially neutralizing the inductance of themodulating coil in the mid-frequency range around 3000 cps. Althoughonly a capacitor was shown across the bias winding in my Patent No.2,189,311, for this purpose I found it desirable to add a resistor inseries with the capacitor. The purpose of this combination is to improvethe frequency response a vibratory element rotatably mounted on saidknife edge, and a line damper mounted on said vibratory element.

2. A galvanometer in accordance with claim 1 in which said armature isformed of two percent vanadium permendur annealed laminationssubstantially .011 inch thick.

3. A galvanometer in accordance with claim 1 in which said magneticfield includes a pair of air gaps of substantially .050 inch and .010inch.

4. A galvanometer comprising a pair of pole pieces, means formagnetizing said pole pieces, an armature having one end held betweentwo opposite ends of said pole pieces and spaced between two other'endsof said pole'pieces, an element mounted on the free end of said armatureand adapted to be torsionally vibrated by movement of said armature, anda line damper mounted on said element.

5. A galvanometer in accordance with claim 4 in which said damperincludes an elongated re- 7 silient member and a fastening member, saidresilient member having one end cemented to said fastening member.

6. A galvanometer comprising means for producing a magnetic field, avibratory armature mounted in said field, a vibratory element adapted tobe driven by said armature, a coil surrounding a portion of saidarmature, said coil being tapped, means for impressing a signal on oneportion of said coil, means for impressing all of said coil with anothersignal, and a reactance circuit in shunt to said last mentioned means.

7. A galvanometer in accordance with claim 6 in which said firstmentioned portion of said coil is substantially one half thereof and isimpressed with an alternating current signal and all of said coil isimpressed with a rectified current signal of said alternating currentsignal.

8. A galvanometer in accordance with claim 6 in which a resilient linedamper is mounted on said vibratory element.

9. A galvanometer comprising means for producing a magnetic field, avibratory armature mounted in said field. a vibratory element adapted tobe driven by said armature, a coil surrounding a portion of saidarmature, means for impressing one portion of said coil with one signal,means for impressing all of said. coil with another signal, and aresistor and condenser connected in series in said means for impressingone portion of said coil with said first mentioned signal.

10. A galvanometer in accordance with claim 9,

in which said first mentioned signal comprises 7 the instantaneousvalues bf a signal and said second mentioned signal comprises theaverage value of said first signal.

GLENN L. DIMMICK.

REFERENCES CITED The following references are of record in the file ofthis patent:

8 unmm sums PATENTS Number Nsmm Date Harrison Oct. 30, 1928 Curtis May9, 1933 Llndenberg Oct. 2, 1934 Curtis Jan. 12, 1937 Kellog Dec. 6, 1938Williams May 13,1941

Badmalefl July 1, 1947

